SQL Newbie Question

Hello Folks

Have a look at this Table:

CREATE TABLE foo(
id serial,
a_name text,
CONSTRAINT un_name UNIQUE (a_name));

Obviously, inserting a string twice results in an error (as one would
expect). But: is there any known possibility to ingnore an errorneous
INSERT like SQLite's "conflict algorithm" (SQLite:"INSERT OR
[IGNORE|ABORT] INTO foo [...]")?
I tried to use a trigger before INSERT takes place, but it seems that
before firing a trigger the constraints are checked...
Background: I'd like to INSERT a few thousand lines in one transaction,
where some values will be appear twice.

thx in Advance